Fluoropolymers have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, and UV-stability. Fluoropolymers include, for example, homo and co-polymers of a gaseous fluorinated olefin such as tetrafluoroethene (TFE), chlorotrifluoroethene (CTFE) and/or vinylidene fluoride (VDF) with one or more gaseous or liquid comonomers such as hexafluoropropene (HFP) or perfluorovinyl ethers (PVE) or perfluoroallyl ethers (PAVE) or non-fluorinated olefins such as ethene (E) and propene (P).
Homopolymers of tetrafluoroethene (PTFE) are highly resistant materials with a very high service temperature. However, PTFE is not melt processable by standard melt extrusion equipment because of its extremely high melt viscosity. Therefore, various TFE copolymers have been developed that are thermoplastic and have a reduced melt viscosity making them melt-processable by ordinary melt-processing equipment. Examples of such fluoropolymers include the fluoropolymer classes PFA (copolymers comprising TFE and perfluorinated vinyl ethers), FEP (copolymers comprising TFE and HFP), THV (copolymers comprising TFE, HFP and VDF), HTE (copolymers comprising TFE, HFP and E), TFE-E (copolymers of TFE and E), TFE-P (copolymers of TFE and P), PVDF (homopolymers of VDF).
Melt-processable thermoplastic fluoropolymers have been used in the preparation of extruded articles like sheets, layers, tubes etc. or in extrusion coatings like for example in the cable and wire industry. Thermoplastic fluoropolymers typically are melt-extrudable. Melt-extrudable polymers have a melting point, which means they are substantially crystalline materials. During melt-processing of thermoplastic fluoropolymers various melt defects can occur. For example, in extrusion processes the rate of extrusion of a fluorothermoplast is limited to the speed (known as critical shear rate) at which the polymer melt undergoes melt fracture. Melt fracture leads to an undesired rough surface of the extruded article (also referred to in the art as “shark skin”). Therefore, the critical shear rate of thermoplasts is typically determined and indicated in the supplier's data sheets. These defects typically occur only on the surface of the polymer and may be predominantly or exclusively caused by the melt-processing equipment or the condition at which the melt-processing is carried out. Various means are available to increase the critical shear rate of a thermoplast to allow for faster melt-processing rates, which is economically advantageous. One example is the use of additives (processing aids). Other attempts have relied on broadening the molecular weight distribution of the polymers or on using bimodal or multimodal polymer compositions, i.e. polymer compositions with distinct populations of polymers having distinctively different molecular weights. Alternatively, modifying the polymer architecture may also improve the melt-processing of the respective fluorothermoplast. For example, in WO2009/009361, the introduction of long chain branches to substantially linear polymers has been described to lead to improved melt processing and melt processed products, for example films and tubes having more homogeneous surfaces.
Other defects may be present in extruded products that are not (only) surface defects. Such “internal” defects may be caused by polymer fractions within the polymer composition that are believed to form gels during melt processing rather than melt. Such fractions are believed to be polymer coagulates created during the polymerization reaction. Typically these defects appear as substantially spherical, circular or oval inhomogeneities in the extruded product and are referred to in the art as “bubbles” or “fish eyes” or simply as “gel content”. Polymers with high gel content will lead to extrusion products of poor visual appearance, either caused by defects in the extruded polymer or by leading to inhomogeneous distribution of adjuvants likes pigments or fillers in the extrusion product.
Thermoplastic TFE-based fluoropolymers are believed to be prone to have an increased “gel content” with increasing melting points. However, higher melting fluoropolymers are desired in many applications as they allow for higher service temperatures and greater processing windows. For example, high processing temperatures may be required in lamination processes of fluoropolymer sheets to form multi-layer articles or during encapsulation processes using high melting encapsulants to form sealed fluoropolymer products as are often required, for example, in the manufacture of solar modules.
Therefore, there is a need for thermoplastic fluoropolymers, and in particular melt-processed fluoropolymers, having high melting points, for example melting points of at least 170° C. that can be conveniently melt-processed, e.g. melt-extruded, and that can be extruded into films having an increased visual appearance, for example by having a reduced gel content.